Recently, can stock material in which a bi-axially oriented polyester resin film is laminated on a metallic sheet has been developed, and can using the same is on the market. Although the polyester resin covered metallic sheet is widely applicable for can stock material because of its excellent economical efficiency and properties, the orientation of the polyester resin layer after the lamination should be reduced to the extent that can body should not be damaged during forming when the polyester resin covered metallic sheet is applied to cans which are formed by heavy forming process such as drawn and stretch-formed can. On the other hand, when the polyester resin layer after the lamination scarcely has orientation, that is, when it is in an almost amorphous state, formed cans or those packed with some content collide with each other during conveying at high speed continuously, and if one of the cans is dented, a crack will form in the polyester resin layer of the dented portion, resulting in local corrosion of the can. Therefore, the orientation of the polyester resin layer after the lamination should be controlled within an extremely narrow suitable range in order to obtain a polyester resin covered metallic sheet having stable quality. Furthermore, even if the orientation of the polyester resin layer is controlled within a suitable range, when the body of a stretch-formed can, packed with carbonated beverage at a low temperature, is dented during conveying at a low temperature, cracks which are problems for practical use will be caused in the polyester resin layer, and the can will be locally corroded. In short, the polyester resin covered metallic sheet has inferior impact resistance, that at a low temperature in particular, and is hard to be stably produced.
Several attempts have been made to improve the impact resistance at a low temperature. Laid open Japanese patent Hei 7-9616 discloses a polycarbonate resin covered metallic sheet, in which the impact resistance at a low temperature is greatly improved without strict control of the orientation to be within a narrow range while keeping excellent flavor retention ability, which is one of the properties essential for can stock. However, a steep cost down of can has been recently demanded, and additional thinning of the can wall thickness is being requested. Moreover, the development of a resin covered metallic sheet which can be formed into 2 piece can, applied for more corrosive content than that packed in the conventional can, is being required. As a result, a resin film used for a resin covered metallic sheet should have stable adhesion during forming as well as extremely excellent adhesion during forming and impact resistance than conventional one because the can wall part is deeply dented and the cracks, which are fatal defects for the protection of content, are easily caused in the resin film by outer impact since the can wall portion is extremely thinned, and further because it is requested to make possible to pack more corrosive content into it.
The double layered composite resin covered metallic sheet disclosed in Laid open Japanese patent Hei 7-9616 may be suitably applied for can of which can wall portion is thinned to the conventional extent. However, the blended resin layer of the lower layer should concurrently have adhesion to the metallic sheet during forming and impact resistance. It is difficult for the resin of lower layer to blend polyester resin having superior adhesion to a metallic sheet but inferior impact resistance and polycarbonate resin having superior impact resistance but inferior adhesion to a metallic sheet at the most suitable blending ratio in which the adhesion and the impact resistance can concurrently reconcile. Therefore, the blending ratio should be in a certain range in which the properties of both resins are compromised. Thus, it is difficult to apply the doubly layered composite resin covered metallic sheet for the use where the can wall portion is more thinned than usual to reduce the weight of the can unless the can is packed with less corrosive content. For example, when most part of the lower layer is composed of polyester resin so as to have adhesion during forming necessary for the thinning of can wall portion to be thinner than that of a conventional one, the impact resistance necessary for resin film at the can wall portion which is greatly thinned is remarkably decreased.
It is the objective of the present invention to produce a composite resin film and a metallic sheet covered with the composite resin film which can be formed into a can formed by heavy forming having more thinned can wall portion than a conventional one when it is laminated on a metallic sheet, which has excellent flavor retention ability after the thus formed can is packed with content and stored, and which has excellent impact resistance even when the can body is dented by an outer impact before or after the content is packed into it.